<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>mission22</title>
    <style type="text/css">
        .root{
            display: flex;
            padding: 20px;
            width: 1000px;
            height: 300px;
            border: 1px solid #000000;
            margin: 100px auto;
            margin-bottom: 10px;
            justify-content: space-between;
        }
        .child_1{
            display: flex;
            padding: 20px;
            width: 450px;
            height: 260px;
            border: 1px solid red;
            justify-content: space-between;
        }
        .child_2{
            display: flex;
            padding: 20px;
            width: 170px;
            height: 220px;
            border: 1px solid green;
            justify-content: space-between;
        }
        .child_3{
            display: flex;
            padding: 20px;
            width: 35px;
            height: 180px;
            border: 1px solid blue;
            justify-content: space-between;
        }
        input {
            margin-left: 100px;
            width: 50px;
            height: 40px;
            font-size: 20px;
        }
    </style>
</head>
<body>
    <div class="root">
        <div class="child_1">
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
        </div>
        <div class="child_1">
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
            <div class="child_2">
                <div class="child_3"></div>
                <div class="child_3"></div>
            </div>
        </div>
    </div>
    <input type="button" id="pre" value="前序">
    <input type="button" id="middle" value="中序">
    <input type="button" id="last" value="后序">
    <script type="text/javascript" src="task.js"></script>
</body>
</html>